Small Bowel Mucosal Disease: Small Bowel Mucosal

0:00Welcome to Board Pearls. This is episode one of one of the Small Bowel Mucosal Disease chapter, in the Stomach and Small Bowel Disorders module. This episode is what to think about when a biopsy shows enteropathy and celiac has been excluded: the wheat-related symptom syndromes, autoimmune enteropathy, Whipple disease, the drug-induced enteropathies, checkpoint inhibitor enteritis, bacterial overgrowth, and tropical sprue.

0:28Everything here runs through one prerequisite: celiac has to be excluded on a gluten-containing diet before any of the alternatives can be named. The transglutaminase antibody with a total IgA is the first screen, biopsy follows, and a patient who's already gone gluten-free has erased the signal, so you either do a gluten challenge or use HLA testing, where a negative haplotype is what excludes celiac. Only once celiac and IBD are off the table does this differential become the working one.

0:58Start with the wheat-related trio, because the boards test it as a sequencing problem: three conditions share bloating, pain, and loose stools after wheat, but split along three mechanisms with three different workups and diet recommendations. Non-celiac gluten sensitivity is a symptom syndrome appearing within hours to days of gluten, with negative celiac serology, normal or only mildly lymphocytic histology, and no IgE wheat allergy, that improves on a gluten-free diet and recurs on rechallenge. It's confirmed by a structured protocol: several weeks on a strict gluten-free baseline, then a blinded placebo-controlled gluten challenge run as a week on, a week washout, and a week crossover, looking for a meaningful drop in symptoms during the gluten-free phase. The most important shift in thinking here is that the offending component is often not gluten at all: the fermentable carbohydrates in wheat, the fructans, reproduce symptoms more reliably than purified gluten in many of these patients, so a lot of people who improve gluten-free are really responding to fructan reduction, which is why the modern workup tries a low-FODMAP diet first in suspected IBS overlap and waits for a structured challenge before committing someone to a lifelong gluten-free label.

2:13The third and quickest of the trio is IgE-mediated wheat allergy, with symptoms within minutes to hours, an allergic phenotype of hives, angioedema, wheeze, and at the severe end anaphylaxis. The variant to know is wheat-dependent exercise-induced anaphylaxis, where symptoms appear only when wheat is followed by exertion, so the textbook stem is the runner who breaks out in hives and wheezes after lunch on training days. It's diagnosed allergologically with skin prick or wheat-specific IgE, and it's the only one of the three that requires complete wheat avoidance for safety rather than symptom control.

2:49The framework tying these together is FODMAP, the fermentable oligosaccharides, disaccharides, monosaccharides, and polyols, short-chain carbohydrates that resist absorption in the upper small bowel, so they draw water in osmotically and reach the colon where bacteria ferment them into gas, distension, and pain. The practical components are the fructans in wheat, onions, and garlic, the oligosaccharides in legumes, lactose in dairy, excess fructose in certain fruits and honey, and the polyols in sugar-free gum. The low-FODMAP diet is validated in IBS and is the favored dietary answer regardless of celiac status, run in three phases: eliminate all the groups for a few weeks, then reintroduce each separately to find personal triggers, then restrict only those, with dietitian supervision because indefinite blanket restriction is nutritionally harmful and alters the microbiome.

3:46Now autoimmune enteropathy, the diagnosis to consider when the patient looks like celiac but doesn't improve on a gluten-free diet. The mechanism is immune attack on the intestinal lining, with circulating anti-enterocyte antibodies in most cases, and in the pediatric form a defect in regulatory T-cell function. The histology is villous atrophy without strikingly raised lymphocytes, which already separates it from celiac, and the characteristic finding is loss of goblet and Paneth cells with deep crypt apoptosis, so villous atrophy with depleted goblet and Paneth cells in a celiac-negative patient who fails the diet is the signature. The pediatric prototype is IPEX syndrome, whose acronym names it, immune dysregulation, polyendocrinopathy, enteropathy, X-linked, from a mutation in FOXP3, the master regulator of regulatory T cells, so affected boys develop severe secretory diarrhea in the first months of life along with diabetes, thyroiditis, and eczema, usually fatal without a stem cell transplant. The adult form is rarer, with chronic high-output secretory diarrhea and severe malabsorption, often needing parenteral nutrition, and it travels with other organ-specific autoimmunity. Anti-enterocyte antibodies support it but aren't fully specific, since they also appear in olmesartan enteropathy, which is part of why those two are hard to separate. The diagnosis rests on a triad: chronic diarrhea with malabsorption, exclusion of celiac and drug-induced disease, and the characteristic histology, with anti-enterocyte antibodies supporting it.

5:18The differential at this fork is broad, villous atrophy with negative celiac serology and no gluten response opens onto common variable immunodeficiency, autoimmune and drug-induced enteropathy on one branch, and Crohn's, tropical sprue, and Whipple on another, so quantitative immunoglobulins are essential here because CVID presents similarly with chronic diarrhea and recurrent sinopulmonary infections, and the man with a thymoma, low immunoglobulins, and villous atrophy is Good syndrome and CVID, not autoimmune enteropathy. Treatment of autoimmune enteropathy is immunosuppression, because it isn't antigen-driven and doesn't respond to diet, with open-capsule budesonide as the favored first-line, delivering drug to the proximal small bowel where the disease lives and often working even in patients who failed systemic steroids, then immunomodulators and biologics for refractory disease and transplant in selected younger patients. So the serologic test the boards want in a stem of villous atrophy with negative celiac serology, no gluten response, and absent goblet cells is anti-enterocyte antibodies.

6:23Now Whipple disease, a systemic infection by Tropheryma whipplei, an organism that colonizes intestinal macrophages and disseminates to joints, the nervous system, the heart, and lymphatics, in patients who seem to have a subtle defect in clearing it, letting it persist in macrophages and produce the characteristic foamy PAS-positive cells. The teaching trap is that the joint pain precedes the gut symptoms by years, sometimes a decade, so it's missed in the early phase because the migratory large-joint arthralgia looks like a seronegative arthritis. The classic patient is a middle-aged white man with soil or animal exposure who develops chronic diarrhea, weight loss, pain, fevers, and lymphadenopathy on a background of years of arthralgia, and the practical pitfall is starting steroids or a TNF inhibitor for presumed seronegative arthritis, which precipitates flares of the intestinal disease. So polyarthralgia plus weight loss plus diarrhea plus soil exposure should put Whipple high on the list before immunosuppression goes in. The neurologic signs are tested directly: rhythmic convergent eye movements synchronized with chewing are essentially pathognomonic, and other features include cognitive decline and ataxia, with the spinal fluid appearing normal but revealing PAS-positive cells and positive DNA testing. Cardiac involvement is a culture-negative endocarditis, so blood-culture-negative endocarditis with negative serologies for the usual culprits should still trigger testing of valve tissue.

7:52Diagnosis is small-bowel biopsy plus DNA testing, sampling the distal duodenum and jejunum rather than the bulb, showing foamy PAS-positive macrophages filling the lamina propria, but that pattern isn't specific, because the atypical mycobacterial infection in HIV produces a similar foamy picture, so the discriminator is the acid-fast stain: Whipple is acid-fast-negative and the mycobacterium is positive, with DNA testing confirming Whipple. You also test the spinal fluid at diagnosis whether or not there are neurologic signs, because subclinical CNS infection sets the treatment duration. Treatment is two-phased and designed around CNS penetration even when the CNS looks uninvolved, because CNS relapse is the dominant late complication: intravenous ceftriaxone for a couple of weeks to induce, then oral trimethoprim-sulfamethoxazole for a year to maintain, chosen because it crosses into the CNS, with doxycycline plus hydroxychloroquine as the sulfa-allergy alternative, the hydroxychloroquine alkalinizing the macrophage compartment to boost the doxycycline. Immune reconstitution inflammation can complicate the start, especially in patients previously immunosuppressed for misdiagnosed arthritis, managed with steroids.

9:06Drug-induced enteropathy is the unifying answer to villous atrophy without a gluten response in a patient on a recognizable offender, and the practical common ground is that the medication list is the diagnostic key, because stopping the drug reverses it, so scrolling the medication list before ordering more biopsies often beats the differential. Olmesartan sprue is the prototype: this angiotensin-receptor blocker can produce a celiac-like enteropathy with severe diarrhea, weight loss, and villous atrophy, mimicking celiac histologically and even sharing HLA-DQ2 positivity and sometimes anti-enterocyte antibodies, which is exactly why it's confused with autoimmune enteropathy, and it can rarely involve other drugs in the class. The clinical picture is an older patient, a few years into the drug, with severe diarrhea and malabsorption, and the histology sometimes shows a collagen band. Treatment is stopping the drug, with improvement over weeks to months, and no rechallenge, because the withdrawal response is the confirmation. So the favored stem is an older patient on olmesartan with severe diarrhea, weight loss, negative celiac serology, and villous atrophy, where the next step is to stop the drug, not start a gluten-free diet or steroids.

10:19NSAID enteropathy is the next well-tested one, producing erosions, ulcers, and the characteristic diaphragm-like strictures, thin concentric web-like membranes that constrict the lumen, from local and systemic prostaglandin inhibition of the lining's repair, diagnosed well by capsule endoscopy and treated by stopping the NSAID with dilation of symptomatic strictures. Mycophenolate enteropathy appears in transplant and autoimmune patients, causing crypt apoptosis and dropout that closely mimics graft-versus-host disease and rejection, so the differential of apoptotic crypt injury in a transplant patient includes GVHD, CMV, and rejection, each needing its own workup, and management is dose reduction or a switch. Methotrexate causes dose-related mucositis timed to chemotherapy dosing.

11:08Now two opposite mechanisms producing similar diarrhea. Checkpoint inhibitor enteritis follows removal of the brake on T cells, so off-target T cells attack gut epithelium and produce colitis or enteritis indistinguishable from IBD, with the CTLA-4 agents carrying the highest GI toxicity, the PD-1 and PD-L1 agents lower, and combination therapy the highest of all, appearing weeks to months after starting, with the workup excluding infection including C. diff and CMV. Treatment is staged by severity: mild diarrhea is supportive while continuing therapy; moderate means holding the drug and starting oral steroids at about a milligram per kilogram; severe means hospitalization with intravenous steroids and rapid escalation to infliximab or vedolizumab if there's no response within a few days, with permanent discontinuation for the worst or steroid-refractory disease. So the favored sequence is hold the drug, rule out infection, start steroids, escalate to a biologic in refractory disease.

12:09Small intestinal bacterial overgrowth is the opposite, an infectious overgrowth of colonic-type bacteria in the small bowel, defined on aspirate culture at or above ten to the third organisms per milliliter, and its mechanism rests on three failures of small-bowel defense: stomach acid sterilizing the upper gut, motility clearing retained contents through the migrating motor complex, and mucosal immunity backing both up, so anything that impairs one sets it up. The structural causes are diverticula, strictures, blind loops, and altered surgical anatomy; the motility causes are scleroderma, the textbook scenario with dilated hypocontractile bowel, plus diabetic neuropathy and pseudo-obstruction; and the acid-suppressive causes are long-term acid blockers and atrophic gastritis. The picture is bloating, gas, diarrhea, and weight loss, with the diarrhea driven by bacterial bile-salt deconjugation and carbohydrate maldigestion, and the classic lab pattern is a low B12 with a high folate, because bacteria consume B12 while synthesizing folate. Diagnosis is aspirate culture as the gold standard but breath testing in practice, measuring hydrogen after a carbohydrate substrate, with the consensus being a rise of at least twenty parts per million within the first ninety minutes, the early peak indicating small-bowel rather than colonic fermentation, and methane is also measured, since methane-positive overgrowth is associated with constipation. Treatment combines correcting the underlying cause with antibiotics, with non-absorbed rifaximin the favored first-line, and cyclic dosing when a motility or anatomic cause makes recurrence near-certain, plus B12 and vitamin repletion.

13:49Tropical sprue closes the differential, a post-infectious enteropathy following enteric infection in a tropical region that produces villous atrophy, malabsorption, and combined folate-plus-B12 deficiency in a returning traveler or long-term resident of an endemic area. The mechanism appears to involve sustained colonization of the proximal small bowel by enteropathogenic bacteria that disrupt the enterocytes and slow motility, allowing further overgrowth in a self-reinforcing loop, producing partial villous atrophy less severe than celiac but enough to cause malabsorption. Geography and travel are the diagnostic anchors, with the Caribbean, the Indian subcontinent, and Southeast Asia prominent, and onset is often abrupt with the patient able to date it, contrasting with celiac's slow course. The picture is malabsorptive diarrhea with weight loss, and the textbook hematologic finding is a macrocytic anemia from combined folate and B12 deficiency, which is the feature distinguishing it from celiac: celiac's proximal involvement causes folate deficiency but spares B12, while tropical sprue hits both the proximal bowel and the terminal ileum, so both are low. Diagnosis is clinical with a jejunal biopsy showing partial villous blunting and negative celiac serology, with the differential being celiac, which spares B12, and giardia, detected on stool testing. Treatment is folate plus tetracycline, with folate producing improvement in most patients and tetracycline added for several weeks to months in someone from an endemic region, plus parenteral B12. So the favored vignette is a traveler returning from the Caribbean or India with chronic foul-smelling diarrhea, weight loss, glossitis, edema, and low folate and B12 with a megaloblastic anemia, a jejunal biopsy showing partial villous atrophy and negative celiac serology, where the answer is tetracycline plus folate.

15:43So pull it together. When a biopsy shows enteropathy and celiac has been excluded on a gluten-containing diet, the next move is mechanistic. Sort the wheat-related symptoms first: non-celiac gluten sensitivity is the diagnosis of exclusion confirmed by the structured challenge, wheat allergy is identified by skin prick or specific IgE, and low-FODMAP is the first structured dietary trial in suspected IBS overlap. Then read the histology actively: villous atrophy with depleted goblet and Paneth cells and deep crypt apoptosis points to autoimmune enteropathy with anti-enterocyte antibodies, treated with open-capsule budesonide, while foamy PAS-positive macrophages in a middle-aged man with years of arthralgia point to Whipple, confirmed by tissue DNA and treated with ceftriaxone then trimethoprim-sulfamethoxazole, with the acid-fast stain separating it from the mycobacterial mimic. The medication list answers the rest, olmesartan and the other offenders reversing on withdrawal, and checkpoint inhibitor enteritis is the immune-mediated answer in oncology patients, graded with stepwise steroid and biologic escalation. Bacterial overgrowth is the answer when motility, anatomy, or acid suppression has failed the small-bowel defenses, diagnosed by breath testing and treated with rifaximin, and tropical sprue is the post-infectious answer in a returning traveler with combined folate and B12 deficiency, treated with tetracycline plus folate. The differential resolves cleanly when the histology, the medication list, and the exposure history are read in parallel.

17:15Chapter twelve widens the lens from small-bowel mucosa to chronic diarrhea and malabsorption as a whole, organized by the osmotic, secretory, fatty, and inflammatory mechanisms, with the initial stool studies sorting the categories and the commonly missed causes, bile-acid malabsorption and microscopic colitis, anchoring the differential.

17:34For the full chapter, the practice vignettes, and the topic-tagged question bank, head to board pearls dot com. You'll find the rest of the series on Apple Podcasts, Spotify, or wherever you listen to podcasts. That brings us to the end of episode one of one of chapter eleven, and I'll see you in the next one.